1. Field of the Invention
The present invention relates to an ultrasonic motor, more specifically, to improvement in constitution of a stator and a support mechanism thereof.
2. Related Background Art
Ultrasonic motors which can give a large amount of power at low speed drive have been wanted in many industrial fields as preferable actuators. And their constitution and operation principles are well disclosed in many articles in magazines, laid-open patents, and the like.
First, the constitution and operation of a conventional ultrasonic motor will be described.
FIG. 1 is a cross-sectional view of an example of the conventional annular ultrasonic motor.
A rotor R consists of a driven element 1 and a slide member 2 which is bonded to the bottom surface of the driven element 1. A stator S consists of an elastic member 3 and an electromechanical energy conversion element 4 such as a piezoelectric member which is bonded to the bottom surface of the elastic member 3. The periphery of the elastic member 3 is attached to support members 100 and 101 by a flange portion 3a provided in the vicinity of a neutral axis, while the rotor R is pressed to be in contact with the stator S by a pressure mechanism (not shown).
FIG. 2 is an enlarged view of part of cross-section of the stator S of the ultrasonic motor shown in FIG. 1.
On the piezoelectric member 4, electrodes 5 of silver, or the like are formed by printing followed by baking or deposition. The piezoelectric member 4 is bonded to the elastic member 3 by an adhesive material layer 6'.
FIG. 3 is a view illustrating arrangement of the electrode formed on the piezoelectric member 4 shown in FIG. 1.
Electrodes 4A, 4B, 4C and 4M polarize the piezoelectric member 4 as well as apply electric power to drive the ultrasonic motor. Signs "+" and "-" in this figure indicate directions of polarization of respective electrodes 4A, 4B, 4C and 4M.
The input electrodes 4A and 4B are electrode groups consisting of electrodes having peripheral length of .lambda./2 and having spatial phases different from each other by .lambda./4. The monitor electrode 4M having length of .lambda./4 is generally used to detect condition of a travelling oscillatory wave (described later) which is generated in the elastic member 3. The electrode 4C, which is generally called a common electrode, is used as ground.
The piezoelectric member 4 is bonded to the elastic member 3 as described above. By applying AC signals having phases different from each other by .pi./2 to respective electrode groups 4A and 4B formed on the piezoelectric member 4, the travelling oscillatory wave is generated in the elastic member 3, thereby driving the rotor R pressure-contacted against the stator S.
Though the above-mentioned conventional ultrasonic motor can give a large amount of power at low speed drive and can be applied in many ways, drive efficiency is low. More specifically, while drive efficiency of a DC motor is typically about 80%, the maximum drive efficiency of an ultrasonic motor is only about 30-40%.
Bad drive efficiency considerably restricts allowable drive time of, for example, various kinds of apparatus using batteries as their power sources.
Further, of electric power applied to the ultrasonic motor, the part which can not be taken out as driving force is consumed to generate heat, thereby rising temperature of the stator. Accordingly, the operation of the ultrasonic motor is restricted when temperature rises beyond an allowable point. As a result, the application of the ultrasonic motor is limited.
Furthermore, temperature rising of the stator may change the characteristics of the ultrasonic motor.